The present invention relates to an apparatus and method for restricting pivoting of industrial vehicles axles.
In the prior art, industrial vehicles, such as forklifts, employ pivotal axles to improve the driving performance of the vehicle. For example, a forklift may have a pivotal rear axle, which is coupled to its vehicle body and supported such that the axle is pivotal relative to the body about a longitudinal axis, or roll axis, of the forklift. Such a pivotal axle permits the wheels of the forklift to follow the road surface and maintain stability. However, pivoting of the axle may result in excessive leaning of the forklift if the vehicle changes directions while moving at a high velocity when carrying a load that is heavy or held at a high position. Leaning of the forklift displaces the vehicle's center of gravity laterally (to the left or to the right) and destabilizes the vehicle. To solve this problem, an apparatus for temporarily restricting pivoting of the axle may be employed. The apparatus temporarily restricts pivoting of the axle when a condition that may decrease the lateral stability of the vehicle occurs.
A known apparatus for restricting pivoting of the axle determines the lateral stability of the forklift from the lateral acceleration and the yaw acceleration of the vehicle. If the vehicle is unstable, the apparatus restricts pivoting of the axle when the forklift turns.
The steps performed by the apparatus for restricting pivoting of the axle will now be described with reference to the graph of FIG. 10. The graph illustrates fluctuation of the yaw acceleration .DELTA..omega./.DELTA.t and the lateral acceleration g of the forklift when the vehicle is first driven straight at a constant velocity, then turned at an angle, and subsequently steered to travel straight again.
As shown in FIG. 10, when the vehicle starts to turn, the yaw acceleration .DELTA..omega./.DELTA.t and the lateral acceleration g begin to increase concurrently. Once the turning radius decreases, the yaw acceleration .DELTA..omega./.DELTA.t beings to increase. In this state, the lateral acceleration g of the vehicle increases. As a result, the leaning amount, or tilting amount, of the vehicle body increases. The restriction apparatus locks and restricts further pivoting of the axle to prevent further leaning of the vehicle when the yaw acceleration .DELTA..omega./.DELTA.t reaches a limit value Y.sub.D. Thus, the tilting of the body is limited when the vehicle is turned.
When the yaw acceleration .DELTA..omega./.DELTA.t reaches its peak and starts to decrease, the lateral acceleration g gradually levels off. As the steering angle of the vehicle becomes constant, the value of the yaw acceleration .DELTA..omega./.DELTA.t falls to zero and the lateral acceleration g becomes constant. However, if the value of the lateral acceleration g is large, the vehicle body leans excessively and lowers the lateral stability of the forklift. Furthermore, the lateral acceleration g may continue to increase even if the yaw acceleration .DELTA..omega./.DELTA.t starts to decrease after reaching its peak value. Therefore, the restriction apparatus also restricts pivoting of the axle when the lateral acceleration g reaches a limit value G.sub.A. Accordingly, pivot restriction of the axle is carried out in accordance with the yaw acceleration .DELTA..omega./.DELTA.t and the lateral acceleration g when the forklift is turned.
If the forklift is steered to travel straight again, the lateral acceleration g decreases and becomes lower than the limit value G.sub.A. Since the forklift is steered in the opposite direction to travel straight forward, the yaw acceleration .DELTA..omega./.DELTA.t decreases and becomes negative. When the lateral acceleration g falls below the limit value G.sub.A, and the negative yaw acceleration .DELTA..omega./.DELTA.t becomes higher than the negative limit value -Y.sub.D after reaching its peak, the forklift becomes stable. Thus, the restriction apparatus permits pivoting of the axle.
However, when the forklift is turned, the tilting speed and amount of the vehicle body are determined by the vehicle's center of gravity in addition to the yaw acceleration .DELTA..omega./.DELTA.t and the lateral acceleration g, which are determined by the velocity and turning radius of the vehicle. In other words, the tilting speed and the amount of tilting of the body vary in accordance with the vehicle's center of gravity, which is determined by the weight and vertical position of the carried load, even under the same velocity and turning radius. If the center of gravity is located at a relatively high position, the tilting speed of the body increases when the forklift is turned. Furthermore, the maximum amount of tilting of the body increases even if the lateral acceleration g is the same.
On the other hand, if the center of gravity is located at a relatively low position when the forklift is turned, the tilting speed of the body becomes slower and the maximum amount of the tilting of the body becomes smaller even if the lateral acceleration g is the same. Hence, the amount of tilting of the body when the yaw acceleration .DELTA..omega./.DELTA.t reaches the limit value Y.sub.D varies in accordance with the position of the center of gravity. Accordingly, the lateral stability of the forklift when changing directions varies in accordance with the position of the vehicle's center of gravity.
If the forklift turns with its center of gravity located at a low position, for example, if the carried load is light or held at a low position, the limit value Y.sub.D related with the yaw acceleration .DELTA..omega./.DELTA.t can be determined such that pivoting of the axle is restricted when the amount of tilting of the body becomes relatively large. On the other hand, if the carried load is heavy or lifted to a high position, the limit value Y.sub.D can be determined such that pivoting of the axle is restricted when the amount of tilting of the body is still relatively small to stabilize the forklift. Since these two conditions do not overlap each other, a value optimal for both conditions cannot be selected as the limit value Y.sub.D. Therefore, the limit value Y.sub.D is set at a value that guarantees stability when the load is heavy or held at a high position. This also guarantees stability when the forklift turns while carrying a load that is light or held at a low position.
The yaw acceleration .DELTA..omega./.DELTA.t varies in accordance with the vehicle velocity and the steering speed, which determine the turning radius. Therefore, an operator of the forklift has a tendency to steer the forklift slowly and carefully when the load is heavy or lifted to a high position. As shown by the dashed line in FIG. 10, in such case, the maximum value of the yaw acceleration .DELTA..omega./.DELTA.t may not reach the limit value Y.sub.D.
As a result, if the forklift turns while carrying a load that is heavy or lifted to a high position, the yaw acceleration .DELTA..omega./.DELTA.t may not restrict pivoting of the axle at an early stage. Although pivoting would be restricted at a later stage when the lateral acceleration g exceeds the limit value G.sub.A (as shown by the dashed line in FIG. 10), the amount of tilting of the body would already have become relatively large. In such case, pivoting of the axle is not restricted when the amount of tilting of the body is still small if the restriction apparatus relies on the yaw acceleration .DELTA..omega./.DELTA.t.